[PULL,03/26] hw/display/bcm2835_fb: fix fb_use_offsets condition

Commit Message

Peter Maydell July 18, 2024, 1:20 p.m. UTC

From: SamJakob <[e-mail]>

It is common practice when implementing double-buffering on VideoCore
to do so by multiplying the height of the virtual buffer by the
number of virtual screens desired (i.e., two - in the case of
double-bufferring).

At present, this won't work in QEMU because the logic in
fb_use_offsets require that both the virtual width and height exceed
their physical counterparts.

This appears to be unintentional/a typo and indeed the comment
states; "Experimentally, the hardware seems to do this only if the
viewport size is larger than the physical screen".  The
viewport/virtual size would be larger than the physical size if
either virtual dimension were larger than their physical counterparts
and not necessarily both.

Signed-off-by: SamJakob <[e-mail]>
Message-id: [e-mail]
Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>
Signed-off-by: Peter Maydell <[e-mail]>
---
 hw/display/bcm2835_fb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/hw/display/bcm2835_fb.c b/hw/display/bcm2835_fb.c
index e40ed2d2e18..650db3da82c 100644
--- a/hw/display/bcm2835_fb.c
+++ b/hw/display/bcm2835_fb.c
@@ -145,7 +145,7 @@  static bool fb_use_offsets(BCM2835FBConfig *config)
      * viewport size is larger than the physical screen. (It doesn't
      * prevent the guest setting this silly viewport setting, though...)
      */
-    return config->xres_virtual > config->xres &&
+    return config->xres_virtual > config->xres ||
         config->yres_virtual > config->yres;
 }